Wireless communications receivers and squelch functions are well known in the art. Squelch functions generally allow a receiver to selectively pass on whatever signal it is receiving to a user, often in an audible format. Viewed from the opposite perspective, when the receiver is not receiving a desirable signal the receiver will be squelched and thus avoid the user annoyance or unreliable reception associated with a radio signal that is too weak or distorted to be useful.
Because of certain technical limitations and subjective user requirements squelch functions may have various characteristics and features. Among such characteristics and features, typically, are a squelch sensitivity, a tight squelch limit, and a unit specific, often user determined, squelch threshold setting. The squelch sensitivity represents the lowest received signal quality that the squelch function can reliably identify as desirable. Practically speaking signal quality is usually synonymous with signal strength for fixed received signal characteristics, including for example modulation parameters.
Similarly, the tight squelch limit represents the opposite end of the spectrum of received signal quality. It will be established by either the technical limitations of the squelch function as a quality level just slightly less than the best signal quality the squelch function can discern or alternatively a quality level beyond which there is little if any debate about the desirability of the signal. In the latter case the tight squelch limit is established to make sure the user does not miss a desirable received signal regardless of the squelch threshold setting. As this suggests the squelch threshold setting is that quality level which when exceeded by the received signal quality results in the squelch function causing the receiver to un-squelch or open. Ordinarily this setting lies somewhere between the squelch sensitivity and tight squelch limit and may depend on any number of imponderables such as subjective user requirements or the typical operating environment of the unit, etc.
While the above was generally acceptable it unfortunately includes a perhaps historically but no longer justified supposition that received signal characteristics which affect squelch performance parameters are largely invariant. Such characteristics may include various modulation parameters like deviation, bandwidth, modulation energy distribution vs frequency and the like. A given receiver subjected to in turn an analog voice transmission, a data communications, a secured analog voice transmission, and etc. will encounter a number of possibly critical variations in such characteristics. Increasing demands placed on today's and likely tomorrow's systems and equipment promises to make variations in such characteristics the role rather than the exception.
The net of all this is that unlike current receiver squelch functions, when modulation parameters change optimum squelch characteristics should also change. This is particularly important for the tight squelch limit. If this characteristic is not modified to correspond with a modulation change the squelch threshold setting can be higher than the best received signal quality that a squelch function is able to discern. This ultimately may result in a user of the receiver missing an undeniably acceptable received signal.
Clearly them is a need for a communications receiver with an adaptive squelch system wherein squelch characteristics vary in accordance with received signal characteristics.